Conventionally, a power output system for a hybrid vehicle is known which includes, for example, an engine, a motor, and a planetary gear mechanism including a sun gear, a ring gear, a plurality of planet gears which mesh with the sun gear and the ring gear and a planet carrier which supports the plurality of planet gears (for example, refer to Patent Literature 1).
As FIG. 63 shows, in a power output system 500 described in Patent literature 1, a primary motor 504 as a generator is connected to a sun gear 502 of a planetary gear mechanism 501, an engine 506 is connected to a carrier 505, and drive shafts 508 are connected to a ring gear 507. By this arrangement, torque of the engine 506 is divided between the ring gear 507 and the sun gear 502 by the planetary gear mechanism 501. The partial torque divided to the sun gear 507 is transmitted to the drive shafts 508. In the power output system 500 described in Patent Literature 1 above, part of the torque of the engine 506 is divided to the drive shafts 508, and therefore, a secondary motor 509 is connected to the ring gear 507 to assist in transmitting torque to the drive shafts 508.